The present invention relates generally to hydraulic clutches and, more particularly, to a hydraulic clutch suitable for use in, for example, an automatic transmission of infinitely variable speed type for automobile vehicles.
A hydraulic clutch for use in combination with an automatic transmission of infinitely variable speed type for automotive vehicles is known, e.g. from U.S. Pat. No. 3,171,524 which employs a plurality of friction plates arranged to be urged against each other directly by a centrifugal force produced by a centrifugal governor. In the hydraulic clutch of this kind, however, the centrifugal governor has to inevitably be designed large in size, in order to obtain required transmission torque. On the other hand, another conventional hydraulic clutch, which is known, e.g. from U.S. Pat. No. 3,690,429, is constructed such that hydraulic pressure from a hydraulic fluid source is controlled to directly control the urging force applied to the friction plates. This necessitates provision of an exclusive, hydraulic pressure control mechanism outside a clutch body, making the hydraulic fluid source or pump large in size, as well as making the control system complicated in structure.
The assignee of the present application has proposed, in Japanese Provisional Patent Publication (Kokai) No. 61-278630, a hydraulic clutch which can obtain sufficient urging force applied to the friction plates, in spite of being simple in construction and compact in size. The hydraulic clutch comprises an input rotating member rotatable in unison with an output shaft of an engine, an output rotating member to which an output of the engine is transmitted through the input rotating member, a plurality of friction plates interposed between the input and output rotating members in such a manner as to be engageable with and disengageable from each other, torque being transmitted from the input rotating member to the output rotating member through the friction plates when engaged, and the torque being prevented from being transmitted from the input rotating member to the output rotating member through the friction plates when disengaged, urging means for urging the friction plates in such a direction as to enable the torque to be transmitted between the input and output rotating members, a hydraulic fluid source generating hydraulic pressure of a value depending upon rotational speed of the output shaft, a hydraulic pressure chamber defined between the urging means and the output rotating member and connectable to the hydraulic fluid source by a passageway having a restriction provided therein, pressure regulating valve means operable to cause the hydraulic fluid to leak from the hydraulic pressure chamber to a low pressure side when the rotational speed of the output shaft of the engine is below a predetermined value and operable to prevent the hydraulic fluid from leaking from the hydraulic pressure chamber to the low pressure side when the rotational speed of the output shaft of the engine is above the predetermined value, control means for controlling the pressure regulating valve means in response to the rotational speed of the output shaft of the engine, and restraining means for restraining the operation of the pressure regulating valve means due to the control of the control means, on and after a specific point during a rotational speed increasing period of the input shaft from the clutch-in rotational speed to the clutch-stall rotational speed.
In the above-described clutch, as shown in FIG. 1 of the accompanying drawings, the engagement and disengagement of the clutch are controlled by the hydraulic pressure depending upon a ratio between flow rate of the hydraulic fluid flowing into the hydraulic pressure chamber 102 determined by the restriction 101 interposed between the hydraulic fluid source 100 and the hydraulic pressure chamber 102, and flow rate of the hydraulic fluid flowing into a hydraulic fluid tank 104 determined by the pressure regulating valve 103 interposed between the hydraulic pressure chamber 102 and the tank 104. FIG. 2 shows a torque transmission characteristic of the clutch equipped with such control means, i.e. a torque characteristic of a drive shaft of the automatic transmission of infinitely variable speed type. In FIG. 2, the abscissa and ordinate represent, respectively, the rotational speed of the engine, i.e. the engine output shaft and the torque transmitted through the clutch. As the engine starts and the rotational speed of the output shaft thereof reaches a predetermined value, the hydraulic pressure within the hydraulic pressure chamber 102 increases, as indicated by a segment a in FIG. 2, to bring the clutch into the engagement. In this state, the force acting upon a cam plate, not shown, by the centrifugal force produced by a control means or governor mechanism, not shown, is lower than that acting upon the cam plate by the hydraulic pressure within the hydraulic pressure chamber 102. Accordingly, the pressure regulating valve 103 is moved by the hydraulic pressure while urging the cam plate, and the pressure within the hydraulic pressure chamber 102 increases with increase in the engine rotational speed, so that the torque transmitted through the clutch increases. As the engine rotational speed further increases, the movement of the pressure regulating valve 103 permits a discharge port thereof, not shown, to be opened, at a point d in FIG. 2, to cause the hydraulic pressure to leak from the hydraulic pressure chamber 102 to the low pressure side through the discharge port. Thus, the pressure within the hydraulic pressure chamber 102 decreases, and the torque transmitted through the clutch decreases as indicated by a segment b. However, as the engine rotational speed further increases and reaches a point just before a point e in FIG. 2, the centrifugal force produced by the governor mechanism increases to overcome the force acting upon the cam plate by the hydraulic pressure. Therefore, the pressure regulating valve 103 is urged to move against the hydraulic pressure in the direction opposite to the above-mentioned direction, to close the discharge port at the point e. Closure of the pressure regulating valve 103 causes the hydraulic pressure to be supplied to the hydraulic pressure chamber 102, and the pressure within the chamber 102 rapidly increases to cause the clutch to engage.
Once the pressure regulating valve 103 is closed (point e in FIG. 2, point A in FIG. 3), however, the hydraulic pressure restricted by the restriction 101 is supplied to the hydraulic pressure chamber 102. Therefore, the hydraulic pressure does not immediately rise to a value required to engage the clutch, i.e. a point C in FIG. 3. so that the force acting upon the clutch piston by the hydraulic pressure is maintained balanced with the force of the spring urging the clutch piston against the hydraulic pressure, for a predetermined time duration T, and the clutch piston is kept stationary. As the hydraulic pressure reaches a point B after the lapse of the predetermined time duration T, the force urging the clutch piston against the force of the spring overcomes the force of the spring, so that the hydraulic pressure abruptly increases from the point B to the point C. This abrupt increase in the hydraulic pressure causes shocks at the engagement of the clutch.